SPID#: 33 The objectives were to identify populations of two New World monkey species, and obtain preliminary data for long-term studies of reproduction and ecology. The specialized genitalia of spider monkeys and howler monkeys (genera Ateles and Alouatta Ceboidea) closely resemble those of apes (Hominoidea); Ateles resembles a monogamous gibbon (Hylobates H. concolor) and Alouatta is most similar to the polygamous chimpanzees (Pan). Evolutionary hypotheses concerning the functional relationships among mating strategy and reproductive anatomy, physiology, and social behavior, which have been developed in comparative investigations of the apes, can be further tested in these ceboid species. Furthermore, field studies enable hypotheses to be evaluated in the context of the species' natural daily activity patterns, their ecology, their population dynamics, and their foraging strategies. Aspects of natural and sexual selection, as well as species specificity, can be investigated. This work has proceeded in collaboration with a number of conservation efforts in Belize including those of the Belize Center for Environmental Studies, and of the Golden Stream Initiative. The latter initiative is aimed at preserving one of the last areas of intact, lowland forest in northern Central America. Apart from well established ethical considerations, this conservation effort is required as long term study necessitates stability for some of the study populations which are under increasing threat from human activity. A central effort was focused on delineating and establishing a large regulated biological corridor of over 3500 km2 that would, in conjunction with the establishment of a Private Reserve, include five of the six study sites. Conservation is a mission and an obligation of the Yerkes Primate Center, and the effort in Belize is one of the ways in which this obligation can be met. Field work in 1995 in Belize, Central America, was completed for six local populations of monkeys [1 and 2] Alouatta at high and low populations densities at low elevation sites [>100 m.] (similar forest structures and environmental conditions); [3 and 4] Alouatta at low population densities at medium (200 - 400 m) and high (550 - 750 m) elevations (different forest structure and distinctive energetic requirements); [5 and 6] Ateles at similar population densities at medium and high elevations. The distinctions found in the density of Alouatta populations were striking with some existing at densities as high as about 80-100 individuals/km2, and others at densities a tenth of this or less. These populations provide a naturally occurring experimental devise to test hypotheses about mate acquisition, mate choice, social behavior and regulatory mechanisms, and the flexibility of mating strategy to accommodate the demands imposed by inter-individual and inter-troop spacing and constraints on food resources. The medium elevation site where Ateles vellerosus was found is extraordinarily accessible given the rarity of the species and its restriction to isolated remnants of mature forest. The species is vulnerable to extinction consequent to insularization, its low reproductive rate, and its low genetic diversity; a Yellow Fever epidemic 30-40 years ago nearly extinguished A.v. yucatanensis in Belize and caused a low effective population size. A program was initiated to protect the species most critical habitats in conjunction with the research effort.